How do I create mist easily?
Zackarus, there are at least 3 methods to create mist: 1 - Cube with Volume Scatter material, 2 - Mist in Compositing, 3 - Fake 3D mist with a plane.

The first method, Volume Scatter, is the most basic, but it's a little bit heavy if you have a slow computer :


Create a cube that contains your scene + camera.
In the Properties panel > Data > Display > Maximum Draw Type > choose Wire so that your cube is transparent in Solid display.
Give your cube a material with a Volume Scatter node plugged into the Volume socket of the Material Output node. Keep the Density value low so that the mist is not too heavy (here: 0.04).




Second method, with the Compositing, tuto here: https://www.youtube.com/watch?v=CxNeMMZs_sM

Third method, fake 3D mist with a plane, answer here: How to get mist only at horizon line?